Context: Circumstellar shells around AGB stars are built over long periods of time that may reach several million years. They may therefore be extended over large sizes ( 1 pc, possibly more), and different complementary tracers are needed to describe their global properties. Aims: We set up a program to explore the properties of matter in the external parts of circumstellar shells around AGB stars and to relate them to those of the central sources (inner shells and stellar atmospheres). Methods: In the present work, we combined 21-cm H I and CO rotational line data obtained on an oxygen-rich semi-regular variable, RX Lep, to describe the global properties of its circumstellar environment. Results: With the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile is parabolic and implies an expansion velocity of 4.2 km s-1 and a mass-loss rate 1.7 × 10-7 M&sun yr-1 (d = 137 pc). The H I line at 21 cm was detected with the Nançay Radiotelescope on the star position and at several offset positions. The linear shell size is relatively small, 0.1 pc, but we detect a trail extending southward to 0.5 pc. The line profiles are approximately Gaussian with an FWHM 3.8 km s-1 and interpreted with a model developed for the detached shell around the carbon-rich AGB star Y CVn. Our H I spectra are well-reproduced by assuming a constant outflow (dot{M} = 1.65 × 10-7 M&sun yr-1) of 4 × 104 years duration, which has been slowed down by the external medium. The spatial offset of the H I source is consistent with the northward direction of the proper motion measured by Hipparcos, lending support to the presence of a trail resulting from the motion of the source through the ISM, as already suggested for Mira, RS Cnc, and other sources detected in H I . The source was also observed in SiO (3 mm) and OH (18 cm), but not detected. Conclusions: A detached shell, similar to the one around Y CVn, was discovered in H I around RX Lep. We also found evidence of an extension in the direction opposite to the star proper motion. The properties of the external parts of circumstellar shells around AGB stars should be dominated by the interaction between stellar outflows and external matter for oxygen-rich, as well as for carbon-rich, sources, and the 21-cm H I line provides a very useful tracer of these regions. eprintid: arXiv:0809.3265